In the exploitation of deep sea oil reserves, fluid conduits called conduits of catenary configuration, commonly know in the oil industry as Steel Catenary Risers are utilized. These conduits are placed at the upper part of the underwater structure, that is, between the water surface and the first point at which the structure touches the sea bed and is only one part of the complete conduction system.
This canalization system is essentially made up of conduit tubes, which serve to carry the fluids from the ocean floor to the ocean surface. At present this tubing is made of steel and is generally joined together through welding.
There are several possible configurations for these conduits one of which is the asymmetric catenary configuration conduit. Its name is due to the curve which describes the conducting system which is fixed at both ends (the ocean bottom and the ocean surface) and is called a catenary curve.
A conduit system such as the one described above, is exposed to the undulating movements of the waves and the ocean currents. Therefore the resistance to fatigue is a very important property in this type of tubing, making the phenomena of the welded connections of the tubing a critical one. Therefore, restricted dimensional tolerances, mechanical properties of uniform resistance and high tenacity to prevent cracking in the metal base as well as in the heat affected zone, are the principle characteristics of this kind of tubing.
At the same time, the fluid which circulates within the conduit may contain H2S, making it also necessary for the product to be highly resistant to corrosion.
Another important factor that should be taken into account is that the fluid which will be carried by the conduit is very hot, making it necessary for the tubes that make up the system to maintain their properties at high temperatures.
Also, the medium in which the tubes must sometimes operate implies maintaining its operability even at very low temperatures. Many of the deposits are located at latitudes with very low temperatures, making it necessary for the tubing to maintain its mechanical properties even at these temperatures.
Because of the afore described concepts and due to the exploitation of reserves at greater depths, the oil industry has found it necessary to use alloys of steel which allow for the obtaining of better properties than those used in the past.
A common practice used to increase the resistance of a steel product is to add alloying elements such as C and Mn, to carry out a thermal treatment of hardening and tempering and to add elements which generate hardening through precipitation such as Nb and V. However, the type of steel products such as conduits not only require high resistance and toughness, but also other properties such as high resistance to corrosion, and high resistance to cracking in the metal base as well as in the heat affected zone once the tubing has been welded.
It is a well known fact that the betterment in some of the properties of steel means determents in other properties, making the challenge to be met the obtaining of a material which provides an acceptable balance among the various properties.
Conduits are tubes that, like conduit tubing, carry a liquid, a gas or both. Said tubing is manufactured under norms, standards, specifications and codes which govern the manufacturing of conduction tubes in most cases. Additionally, this tubing characterized and differentiated from the majority of standard conduction tube in terms of the range of chemical composition, the range of restricted mechanical properties (yielding, stress resistance and their relationship), low hardness, high toughness, dimensional tolerances restricted by the interior diameter and criteria of severe inspection.
The design and manufacturing of steel used in heavy gauge tubing, presents problems not found in the manufacturing of tubes of lesser gauge, such as the obtaining of the correct hardening, a homogeneous mixture of the properties throughout the thickness and a homogeneous thickness throughout the tube and a reduced eccentricity.
Still another more complex problem is the manufacturing of heavy gauge tubing which fulfills the correct balance of properties required for its performance as a conduit.
In the state of the art, for the manufacturing of tubing to be used as conduits, we may refer to the document EP 1182268 of MIYATA Yukio and associates, which discloses an alloy of steel used for manufacturing conduction or conduit tubing.
In this document the effects of the following elements are disclosed: C, Mo, Mn, N, Al, Ti, Ni, Si, V, B and Nb. Said document indicates that where the contents of carbon is greater than 0.06%, steel becomes susceptible to cracking and fissures during the tempering process.
This is not necessarily valid, since even in heavy gauge tubes, and maintaining the rest of the chemical composition the same, no cracking is observed up to carbon contents of 0.13%.
Furthermore, upon trying to reproduce the teachings of MIYATA and associates, it may be concluded that a material with a maximum range of carbon of 0.06% could not be used for the manufacturing of heavy gauge conduit since C is the main element which promotes the hardenability of the material and it would prove very costly to reach the high resistance required through the addition of other kinds of elements such as Molybdenum which also promotes, given a certain content, detriment in the toughness of the metal base as well as in the heat affected zone and Mn which promotes problems of segregation as we shall see in more detail later on. If the content of carbon is very low, the hardenability of the steel is affected considerably and therefore a thick heterogeneous a circular structure in the half-value layer of the tube would be produced, deteriorating the hardenability of the material as well as producing an inconsistency in the uniformity of resistance in the half-value layer of the tubing.
Furthermore, in the MIYATA and associates document, it is shown that the content of Mn improves the toughness of the material, in the base material as well as in the welding heat affected zone. This affirmation is also incorrect, since Mn is an element which increases the hardenability of steel, thus promoting the formation of martensite, as well as promoting the constituent MA, which is a detriment to toughness. Mn promotes high central segregation in the steel bar from which tubing is made, even more in the presence of P. Mn is the element with the second highest index of segregation, and promotes the formation of MnS inclusions, and even when steel is treated with Ca, due to the problem of central segregation of Mn above 1.35%, said inclusions are not eliminated.
With contents of over 1.35% Mn a significant negative influence is observed in the susceptibility to hydrogen induced cracking known as HIC. Therefore, Mn is the element with the second most influence on the formula CE (Carbon equivalent, formula 11W), with which the value of the content of final CE increases. High contents of CE imply welding problems with the material in terms of hardness. On the other hand, it is know that additives of up to 0.1% of V allow for the obtaining of sufficient resistance for this grade of heavy gauge tubes, although it is impossible to also obtain at the same time high toughness.
One known way in which said tubes are manufactures is through the process of pilger mill lamination. If it is true that by way of this process high gauges of tubes may be obtained, it is also true that good quality in the surface finish of the tube is not obtained. This is because the tube being processed through pilger mill lamination acquires an undulated and uneven outer surface. These factors are prejudicial since they may lessen the collapse resistance which the tube must possess.
On the other hand, the coating of tubes which do not have a smooth outer surface is complicated, and also the inspection for defects with ultrasound becomes inexact.
Steel which may be used to manufacture tubes for conduction systems with catenary configurations, heavy gauges, high stress resistance and low hardenability, and which complies with the requirements of toughness to fissures and resistance to the propagation of fissures in the heat affected zones (HAZ), and resistance to corrosion, necessary for these types of applications has yet to be invented since without the quality of heavy gauges, the simple chemical composition and heat treatment do not allow for the obtaining of the characteristics necessary for this type of product.
The precedents which have been analyzed indicate that the problem has not yet been integrally resolved, and that it is necessary to analyze other parameters and possible solutions in order to reach a complete understanding.